The present invention generally relates to thread forming screws and methods of forming same, and more specifically relates to a fastener, such as a thread forming screw, at least a portion of which has a minimum surface hardness of HRC 56 (Rockwell Hardness C-scale), and to a method of forming such a thread forming screw.
Currently, the industry recommends that thread forming screws be able to cold form threads in a workpiece having a hardness of Rockwell C23 and below. As such, current thread forming screws are sufficiently hard to cold form threads in such workpieces. However, there are some applications where it would be desired to have a thread forming screw be able to form threads in a workpiece which has a hardness which exceeds HRC 23. For example, after High Strength Low Alloy (HSLA) material is formed, such as by punching holes in the material, the hardness in and around the hole area could reach and exceed a Rockwell C40 hardness.
Conventional thread forming screws are not hard enough to successfully cold form threads into workpieces which exceed a Rockwell C23 hardness. For example, some commercially available thread forming screws are processed so that the point and the first three to four full threads are at a Rockwell C45 minimum hardness, and the core of the fastener is at Rockwell C33 to 39 hardness. A thread forming screw such as this is not hard enough to consistently cold form threads into a material which exceeds HRC 23. When driving such a thread forming screw into HSLA material (or any other material) having a hardness of HRC 40 or more, thread collapse occurs. This collapse, or deformation of the threads, causes joint failure. When forming threads in materials such as steel, it is required to have the fastener have a hardness much greater than the material into which the fastener is being threaded, in order to form the threads properly without collapsing.
Because current thread forming screws are not sufficiently hard to cold form threads into workpieces having a hardness which exceeds HRC 23, in such cases it has been necessary to provide mating threads, such as by providing a threaded joint in the workpiece (which requires a drill and tap operation), or by providing weld nuts, loose nuts, a weld boss, threaded inserts, or by providing a similar feature or using a similar method for providing mating threads. These structures/methods require additional operations, hardware and time, resulting in a more costly, time-consuming process.
As such, a market exists for fasteners capable of forming threads in workpieces, such as HSLA material or any other material, which has a hardness which exceeds Rockwell C23.